Located at the
recoil mass spectrometerfocal plane,
this
detector will provide mass identification of the spacially separated
recoil products.
The PSAC will have a 36x10 cm. active area and consists of four
wire planes constructed from 0.32 cm. FR4 printed circuit board. The
boards will be stacked as closely as possible in order to provide a
more uniform electric field and to minimize the gas volume. Approximately
250 volts will be placed on the first wire plane (cathode 1) and third wire
plane (anode) and if necessary for a more uniform field, a fifth wire plane
(cathode 2). The second and fourth wire planes will be operated at ground
potential.
This arrangement should provide a uniform field with the horizontal sensing
wire plane (X) between cathode 1 and the anode and the vertical sensing plane
(Y) immediately following the anode.

All wires will be 20 micron gold--plated tungsten separated by 2 mm.
Isobutane gas will be constantly circulated through the chamber at a pressure
of approximately 3 torr. Commercially available mylar will be used for the
windows and will be supported with wires of 50 micron diameter.
The anticipated transmission of such an arrangement is expected to
be greater than 95%.

Spacial sensitivity will be acheived through time delays of the X and Y
signals. As
an ion passes through the chamber, charge is
induced on the X and Y wire planes. Each wire is electrically connected
to the others through a passive delay line (2 ns delay per wire). The
signals are taken from both ends of the active area and the time difference
between the signals is related to the position of the transversing ion.
The large size of the X plane, requires that the active area be separated
into 2 halves.
In addition to the X and Y signals, signals from the anode and cathode
will also be collected. It is envisioned that one of these signals
will be used to gate any additional detectors firing in coincidence and
classify the event as a recoil event.
Although at such pressures the energy loss signal of the ion is expected
to be small, these signals may be used to discriminate between the
recoils of interest and any scattered beam which may somehow survive
the RMS and strike the PSAC.

The PSAC will be housed in a chamber which is bolted into the larger
focal plane
housing. The chamber will have room for 16 vacuum
feedthroughs using LEMO connectors and two gas feedthroughs. The gas intake
will have a perforated hose attached in order to ensure that the gas is
spread evenly throughout the chamber. The gas will be free flowing and will
not be recirculated. Two 300 l/s turbomolecular vacuum pumps may be positioned
directly under the PSAC in order to maintain good vacuum. Although these
pumps may not be necessary when the experiment is using the PSAC only,
they will be required if an
ionization chamber
is also used.